The Morning Gift by Eva Ibbotson

Twenty-year-old Ruth Berger is desperate. The daughter of a Jewish-Austrian professor, she was supposed to have escaped Vienna before the Nazis marched into the city. Yet the plan went completely wrong, and while her family and fiancé are waiting for her in safety, Ruth is stuck in Vienna with no way to escape. Then she encounters her father’s younger college professor, the dashing British paleontologist Quin Sommerville. Together, they strike a bargain: a marriage of convenience, to be annulled as soon as they return to safety. But dissolving the marriage proves to be more difficult than either of them thought—not the least because of the undeniable attraction Quin and Ruth share. To make matters worse, Ruth is enrolled in Quin’s university, in his very classes. Can their secret survive, or will circumstances destroy their love?